Typically autofocus systems in digital cameras, for capturing still images or video, use a “Through-the-lens” autofocus system that captures a series of 5-20 or more autofocus images taken with a moveable focus lens in different focusing positions. For an autofocus system that includes a variable focus lens with variable focal length or variable optical power for focusing, instead of moving the focus lens, the variable focus lens is adjusted electronically to provide 5-20 or more different focal lengths or optical powers for the autofocus images.
After capture, the 5-20 or more autofocus images are analyzed for contrast to determine the focus lens condition that delivers the image with the highest contrast which is deemed the best focus condition. In the analysis, focus values are generated for each autofocus image based on the level of contrast present. The focus lens is then returned to the focus condition that produced the autofocus image with the highest contrast (highest focus value), or an interpolated position between two or more of the autofocus images, before a final image is captured and stored. This method of autofocusing is known as the “hill climb method” because it generates a sequence of focus values that increase in level until they pass over a peak, i.e., a “hill”.
“Through-the-lens” autofocus systems can be very accurate since they measure focus quality directly from autofocus images captured with the same high quality lens that is used to capture the final image. However “through the lens” autofocus systems can also be very slow due to the many movements of the focusing lens required and the many autofocus images that must be captured and analyzed. This slowness in time-to-focus contributes to the objectionable delay perceived by the user between the time when the capture button is pressed and the image is actually captured, which is known as shutter lag. It is desired to reduce shutter lag.
During video capture, the autofocus images are typically derived from the same series of still images or frames that compose the video segment, consequently, the process of autofocusing causes 5-20 or more out-of-focus frames to be produced in the video each time the scene changes. As a result, during video capture with pan movements of the camera where the scene changes continuously, large portions of the video are actually out of focus. Ideally, the autofocus system would be faster when capturing video as well as still images, and in the case of video capture, each frame would be focused so that the number of out of focus frames is reduced. This is especially important in enabling images from videos to be printed or used in other fashions.
Dual lens rangefinder modules can also provide a fast evaluation of focus conditions. Rangefinder modules can be purchased from Fuji Electric in several models such as the FM6260W. Dual lens rangefinder modules contain two lenses that are separated by a distance along with two matching sensor areas to enable matched pairs of low resolution images to be captured. The matched pairs of low resolution images are then analyzed for correlation between the two images to determine the offset between the two images caused by the separation between the two lenses. The offset information is then used along with the lens separation distance to calculate the distance to the scene by triangulation. The calculated distance to the scene is used to guide the use of the focus lens based on a calibration curve established between the distance to the scene as measured by the dual lens rangefinder module and a series of best focused images as produced by the through the lens autofocus system. The response time of the Fuji FM6260W modules is advertised as 0.004 sec in high sensitivity mode, which is well within the 1/30 sec required for video autofocus. However, the accuracy of dual lens rangefinder modules are typically influenced by changes in the environmental conditions such as changes in the temperature and/or humidity. So that typically these dual lens rangefinder modules are not used independently for autofocus in digital cameras but instead are used as a rough focus adjustment that is supplemented by a through the lens contrast based autofocus system. The problem with the dual lens rangefinder modules is that the calibration between the dual lens rangefinder module and the focus lens setting is not stable within the normal operating environment for digital cameras. Environmental conditions such as changes in temperature and humidity can cause the calculated distance to the scene produced by the dual lens rangefinder module to change by over 10% and in addition, the measured position of the moveable lens in the moveable lens control system is prone to environmentally induced changes as well. In addition, the dual lens rangefinder module adds the additional cost to the camera of the dual lens rangefinder module itself.
In a through-the-lens split aperture device, as described United States Patent Publication US20080002959, a split aperture in the lens system is used to create images that can be interpreted for focus information. The split aperture creates two optical paths for the light passing through the lens to create at least two autofocus images at the sensor. By splitting the optical path at the aperture of the lens system, each of the two optical paths creates a full image without shading but reduced light intensity at the image sensor. By sequentially partially blocking two different portions of the aperture thereby splitting the aperture, two optical paths with different perspectives are created. The difference in perspective between the two optical paths causes the autofocus images to be displaced laterally in proportion to the degree of defocus and direction of defocus for an object in the image. However, the focus resolution of the split aperture method as measured by the number of detectable focus zones is limited by the effective separation achieved between the two optical paths that are created which is approximately 40% of the lens aperture. As lens apertures get smaller, the focus accuracy of this technique is diminished due to a lack of focus resolution, this is particularly true for small image capture devices such as are found in compact digital cameras, cellular telephones, laptop computers and other communication devices.
Therefore, a need exists for autofocus systems to be improved to provide less shutter lag for still capture and fewer out-of-focus frames for video while providing the focus resolution to enable accurate focusing.